In long distance optical transmission systems, it is desirable to launch the highest optical power possible into the transmission fiber link. High power enables the signals to be transmitted over longer distances without the need for additional components such as regenerators, repeaters, and amplifiers, which increase the cost of communication systems. However, with the higher optical launch power, optical fibers exhibit non-linear effects such as stimulated Brillouin scattering (SBS), four-wave mixing (FWM), stimulated Raman scattering (SRS) and self phase modulation (SPM). These non-linear optical effects, whose magnitude depend on the non-linear optical coefficients of the fiber material, fiber length (L) and the laser signal linewidth (Δν), limit the amount of useable launch power into the communication system. These phenomena can degrade the optical signals and increase bit error rates (BERs) for the data transmitted by the system.
The power of the optical signal is also a factor in determining the severity of these non-linear effects. Since the optical power is concentrated into a smaller cross section area in optical fibers, a modest optical power level can produce these nonlinear effects. Among the non-linear effects described above, SBS occurs at the lowest inserted power level in fiber communication systems, commonly using commercial semiconductor distributed feedback (DFB) lasers having narrow linewidths (˜1 MHz). Thus, SBS has been considered as setting a limit on the launched power level in optical fiber systems. The threshold for this effect is on the order of a few milliwatts of optical power for typical single mode optical fibers; below this threshold power, the scattering is a spontaneous process and the scattered light intensity is so low that signal degradation is minimal.
The primary origin of the SBS phenomenon is acousto-optic fluctuations via electrostriction. The pump wave generates acoustic waves through the process of electrostriction. Electrostriction is the tendency of material to compress in the presence of an electrical field. In turn, electrostriction causes a periodic modulation of the fiber refractive index in the form of an optical grating. This pump-induced refractive index grating scatters the signal light through Bragg diffraction. In optical fibers, the SBS travels in a backward direction and is shifted in frequency proportional to both the sound velocity and refractive index of the fiber. The frequency shift, commonly referred to as the Brillouin shift, is given by:                                           v            B                    =                                    2              ⁢                              nV                A                                                    λ              p                                      ,                            Equation        ⁢                                   ⁢                  (          1          )                    where λp, n, VA are the wavelength of the incident pump, the refractive index of the core and the sound velocity of the material, respectively.
The SBS threshold power Pthr is given by:                               P          thr                ≅                                            21              ⁢                              A                eff                                                                    g                B                            ⁢                              L                eff                                              ⁢                      (                          1              +                                                Δ                  ⁢                                                                           ⁢                                      v                    p                                                                    Δ                  ⁢                                                                           ⁢                                      v                    B                                                                        )                                              Equation        ⁢                                   ⁢                  (          2          )                                                  L          eff                =                              1            α                    ⁡                      [                          1              -                              ⅇ                                                      -                    α                                    ⁢                                                                           ⁢                  L                                                      ]                                              Equation        ⁢                                   ⁢                  (          3          )                    where Δνp is the input laser linewidth, ΔνB is Brillouin linewidth, Aeff is the effective core area, Leff is the effective length, and gB is the Brillouin gain of the fiber medium. Additionally, Rayleigh scattering, whose origin is non-propagating density fluctuations, occurs in a backward direction, but the scattering intensity is lower than that of SBS.
For a given length of a fiber, the SBS threshold depends mostly on the linewidth of the laser source and Brillouin linewidth of the medium. The Brillouin linewidth is the linewidth of the backscattered Brillouin light in the frequency domain and is inversely proportional to the acoustic phonon lifetime in the medium.
A known uni-directional WDM fiber optic transmission system 100 without any SBS suppression is shown schematically in FIG. 1. A transmitter 110 generates light signals of multiple wavelengths (λ1 . . . λN) along multiple fibers 112 to a first WDM 114, which allows the multiple wavelengths to travel along a single fiber 116. A booster amp 118 is disposed downstream of the first WDM 114 to amplify the transmitted signal prior to transmission along a fiber 120. Proximate the end of the transmission fiber 120, the signal is transmitted through a pre-amp 122 prior to transmission to a second WDM 124, which splits the multiple wavelengths (λ1 . . . λN) into individual fibers 126 for transmission to a receiver 130. SBS is generated along the fiber 120 and propagates backward, toward the transmitter 110, causing undesirable noise in the system, which degrades the light signals being transmitted from the transmitter 110 to the receiver 130.
FIG. 2 schematically shows an application of a unidirectional system, in a community antenna television (CATV) system 200. An optical signal is generated at a head end 210 and transmitted along a plurality of optical fibers 212 to fiber amplifiers 214 (only one fiber amplifier 214 is shown). A splitter 216 divides the optical signal for transmission along multiple optical fibers 220. Each optical fiber 220 terminates in a fiber node 222 (only one shown for clarity) proximate to an end destination, such as a residence 230. The fiber node 222 converts the optical signal to an electrical signal for transmission along an electrical bus 232, where individual coaxial cables 234 transmit the signal to the residence 230. Without SBS suppression, the fiber launch powers in typical single mode fiber (e.g. SMF-28) are limited by SBS. The upper limit of the optical power in a typical fiber optic CATV system 200 without SBS suppression is, for example, approximately +17 dBm (approximately 50 mW) for a 50 km single mode fiber.
A prior art bi-directional fiber optic transmission system 300 without any SBS suppression is shown schematically in FIG. 3. A first transmitter/receiver 310 transmits a first light signal having a wavelength λF from left to right, through an amplifier 312, to a first WDM 314, along a fiber 316 to a second WDM 320 and to a second transmitter/receiver 330. Simultaneously, the second transmitter/receiver 330 transmits a second light signal having a wavelength λB from right to left, through an amplifier 332, to the second WDM 320, along the fiber 316 to the first WDM 314 and to the first transmitter/receiver 310. The first light signal generates SBS, which travels from right to left and interferes with the second light signal, and the second light signal generates SBS, which travels from left to right and interferes with the first light signal.
To improve upon transmission quality in the systems shown in FIGS. 1-3, several techniques have been demonstrated to suppress SBS in optical transmission systems. Basically, these techniques can be put into two main groups in terms of their approach to the problem. One approach to suppress SBS is based on broadening the laser linewidth via either frequency modulation (FM) or phase modulation (PM). As seen from Eq. (2), broadening the laser linewidth results in a higher SBS threshold. Both direct FM and external PM, which both introduce a predetermined amount of spectral broadening to the laser, have been demonstrated as effective means of suppressing SBS in optical systems.
The direct FM approach uses a dither signal on the laser bias to provide large frequency excursions, usually on the order of 10 GHz. By means of this technique, the SBS threshold has been increased by as much as 15 dB, as disclosed in U.S. Pat. No. 5,329,396 (Fishman et al.). However, direct FM of an injection laser also results in substantial AM, called residual AM, which degrades the system performance, especially for analog transmission systems.
By contrast, the external PM approach avoids the production of the residual amplitude modulation while still suppressing SBS. U.S. Pat. No. 5,566,381 (Korotky et. al.) discloses a 17 dB increase in the SBS threshold by PM modulation of the laser with more than one radio frequency (RF) source. However, in optical transmission systems, this external PM technique typically degrades the dispersion characteristics of the signal due to an excessive increase in linewidth of the laser source.
FIG. 4 shows a schematic diagram of prior art system 400 to suppress SBS. These techniques are based on either a direct frequency modulation of a laser driver, or an external phase modulation of a laser signal. The laser can be a conventional solid-state laser (e.g. DFB) with a wavelength preferably chosen in either optical communication window (1300 nm or 1550 nm). Optical communication windows are wavelengths at which signal losses are minimized. Both techniques provide the linewidth broadening of the laser source. Broadening the optical linewidth of the laser reduces the spectral density of the signal. Thus, the same optical power becomes distributed over a broader range of spectrum, and the SBS threshold increases depending on the broadening ratio. Both direct FM and external PM, which both introduce a predetermined amount of spectral broadening to the laser, have been demonstrated as effective means of suppressing the SBS in optical systems. On the other hand, the external PM approach avoids the production of residual amplitude modulation and also suppresses SBS.
As shown in FIG. 4, a laser 410 transmits an optical signal along a fiber 412. The optical signal is injected into a Mach-Zehnder electro optic modulator (EOM) 420, which modulates the optical signal. The EOM420 has two arms, with one arm having a LiNbO3 crystal which is driven by an electrical signal to generate a phase difference between the two arms. When a phase difference, such as π, is present, the optical signal is blocked and cannot travel through the EOM 420. When no phase difference is present, the optical signal can pass through the EOM 420. The EOM 420, which is driven by an RF signal, provides the PM at the output 422. The linewidth broadening is proportional to the frequency and modulation index of the RF signal. It has been disclosed in U.S. Pat. No. 5,166,821 (Huber) and U.S. Pat. No. 5,420,868 (Chraplyvy et al.), that the SBS threshold can be increased by 5 dB by using a similar optical phase modulator driven with a single frequency sinusoidal signal. But, increasing the SBS threshold further requires a high modulation index, which also requires very high RF drive power. Recently, PM modulation of a laser with more than one RF driver has been shown to provide more linewidth broadening and, therefore, further SBS suppression. Korotky discloses that modulation of a DFB laser signal by means of an EOM, which is driven with four frequencies (70, 245, 858, and 3001 MHz) and a total RF power of 250 mW, may provide a 17 dB increase in the SBS threshold. However, in optical transmission systems, this external PM technique typically results in degrading the dispersion characteristics of the propagating optical signal due to the excessive increase in linewidth of the laser source. Another approach to suppress SBS is based on the modification of the fiber medium properties along the longitudinal direction of the fiber. As noted earlier above, the Brillouin shift is dependent on the medium properties such as refractive index and sound velocity. The parameters that influence the refractive index and the sound velocity in turn are composition of the glass, residual fiber stress, density of the fiber core, and temperature. In order to provide a high SBS threshold, these parameters should be altered in a distributed manner through the fiber.
FIG. 5 shows a schematic of a fiber 500 which represents the method described above by employing various lengths of fibers L1, L2, . . . LN with different material properties νB1, νB2, . . . νBN, respectively, or by continuously modifying the fiber properties of a single fiber that produce different Brillouin shift frequencies. Thus, the resultant Brillouin spectrum of the fiber 500 contains various frequency components without overlapping each other, which provides a Brillouin gain within a broad bandwidth and results in higher SBS threshold. Thus, the Brillouin gain profile would vary along the fiber 500, which avoids the accumulation of the gain within a small bandwidth and results in a broader gain profile and higher SBS threshold. However, this approach is not practical because of the difficulty in manufacturing such fibers and the lack of use in already installed optical fiber systems.